Take out zombies in relative safety by having them bite the shield.
It works against Cerberuses as well, so if you can't fire off a few shots in time,
push the action button (PS4: X, PC: A) to bring up the shield,
and finish them off with a shot (PS4: R2, PC: right trigger) or a melee attack (PS4: R3, PC: right stick button).

All it takes is a single bullet (or none if you perform a melee), which makes it particularly useful for ammo conservation.
It's also effective against enemy Brainer attacks: by forcing your opponent to lock on to a zombie instead of yourself,
you can buy valuable time for a counter attack.

If you keep your shield up for too long however, sooner or later a zombie will bite you instead.
Try bringing up your weapon from time to time to keep the more distant zombies from closing in (this resets the zombie timer).